The sodium/iodide symporter (NIS) mediates iodide transport in the thyroid and lactating breast. Targeted expression of a NIS-expressing transgene in a range of tumor models has promoted successful radioiodine therapy, but this approach is limited by the requirement to successfully achieve gene transfer into the tumor. Activation of endogenous NIS in thyroid cancer by thyroid stimulating hormone (TSH) is the primary tool for effective imaging and treatment of thyroid cancer. Basal NIS expression in MCF7 breast cancer cells is generally low, but is markedly stimulated by retinoic acid (RA). RA treatment results in significant concentration of radioiodine in breast cancer cells and in mouse breast cancer models. We have recently shown that RA stimulation of NIS expression in breast cancer cell lines is retinoic acid receptor (RAR) isoform-specific, mediated predominantly by the RAR(32 isoform, and that dexamethasone enhances induction. Inhibitors of the phosphatidylinositol-3 kinase (PIS kinase) signal transduction pathway completely block RA-induced NIS mRNA and protein expression. The signal transduction pathways in breast cancer cells are distinct from those activated by TSH that stimulate NIS expression in thyroid cancer. Our hypothesis is that unique mechanisms of regulation of NIS in breast cancer, distinct from those in lactating breast or thyroid, can be defined and exploited to selectively deliver radioiodine that will be effective for diagnosis and treatment of some differentiated breast cancer. The following specific aims will be investigated;1. Determine the mechanism of RA-stimulation of the phosphatidylinositol-3 (PIS) kinase signal transduction pathway and downstream mediators of NIS gene expression in breast cancer. We will utilize pharmacologic antagonists, agonists, genetics, and functional proteomics, to map the relevant interacting factors. 2. Define the mechanisms and conditions that optimize NIS gene expression and functional iodide uptake in breast cancer cell lines, compared to thyroid, through maximizing NIS protein expression, subcellular localization, and kinetics. 3. Utilize the defined optimized combinations of NIS gene and protein stimulatory factors in transgenic and xenograft in vivo models to maximally concentrate radioiodine and treat tumors. The findings from this study can potentially be applied to radioiodine imaging and treatment of breast cancer and further understanding the factors that regulate breast cancer growth and differentiation.